磁尾等离子体传输及相关观测现象的研究

The plasma sheet boundary layer (PSBL), an active region in the Earth's magnetotail with field-aligned ion beams often observed, is also a major conduit of earthward energy transport. Various processes have been proposed to explain the presence of PSBL beams, and the majority of these processes are disjoint from the central plasma sheet (CPS) properties adjacent to the PSBL observational site. By analyzing multipoint observations of the PSBL and its nearby CPS, Zhou et al. (2012a) discovered the intrinsic connection between PSBL beams and CPS bursty flows, and proposed that the PSBL ion beams arise from the ion reflection process at dipolarization fronts (DFs) associated with CPS bursty flows. The emerging scenario, therefore, unifies the impulsive transport phenomena in the PSBL and the adjacent CPS. Like any newly-proposed paradigm, the unified PSBL-CPS transport scenario requires to be examined in the framework of energy dispersion signatures. Also, the near-Earth tail energy budget within the context of the new scenario has never been analyzed. These are the main goals of our proposed study. Specifically, we propose to address the following questions: 1. What are the most common patterns of energy dispersion in the PSBL? What are the key factors that can affect the dispersion patterns? Can they be explained by the unified PSBL-CPS transport scenario, and be reproduced by the refined simulation model of ion DF-reflection? Also, what are the conditions when dispersion properties different from the most common ones are observed, and what causes these different patterns? 2. What is the typical energy budget according to the unified PSBL-CPS transport scenario? Does the particle energy converted from electromagnetic energy at DFs match the energy fluxes ahead of the DFs (at both PSBL and CPS)? Does the simulation result agree with observations? To answer this questions, we propose to expand the current event list to include more PSBL ion beam events with simultaneous observations at the adjacent CPS. We propose to carry out statistical and case studies based on multi-point observations to identify the most typical dispersion signatures and energy budgets, which will be compared with simulation results for verification and refinement of the unified transport scenario.

等离子体片边界层是地球磁尾能量传播的一个主要区域，其主要观测特性是沿磁场方向的离子束流。目前已有多种机制试图解释此现象，然而其中绝大多数都与边界层附近的中心等离子体片性质无关。通过多点卫星观测，我们发现了该场向离子束流与其临近的中心等离子体片高速流之间的内在联系，并由此提出该场向束流源自离子在偶极化锋面的反射。这种全新机制统一了等离子体片不同纬度的等离子体传输特征。和所有新机制一样，这个全新的图像还需要进一步的检验和发展。这正是此课题的主要目标：等离子体片边界层最常见的能量色散特征是什么？它们受哪些参数影响？它们能否在这种全新的等离子体传输机制的框架里被解释和模拟？偶极化锋面上的电磁能转化是否同等离子体片高速流及边界层离子束流的动能一致？我们将利用THEMIS数据扩大现有的多点观测事件列表，并用个例分析及统计的方法将观测和模拟结果进行比较，以确认和进一步发展这种全新的磁尾等离子体传输图像。

本项目着力于研究地球磁尾的带电粒子动力学，尤其是当偶极化锋面朝地球方向运动时带电粒子的行为及其与其他磁尾现象（等离子体片边界层离子束流、磁场拓扑结构变化、能量转换与传输等）之间的内在联系。另一方面，这些带电粒子由磁尾注入内磁层后，也可为内磁层中的波和不稳定性提供自由能，从而进一步影响整个磁层的等离子体行为。本项目紧密围绕这一课题，并取得了一系列学术成果。..首先，本项目利用THEMIS卫星数据和数值模拟方法，深入探讨了偶极化锋面对带电粒子的反射与加速作用，并在其基础上详尽分析了其主要过程与后果。这些后果包括：等离子体片边界层粒子束流及其晨昏不对称性（成果1）、偶极化锋面处的能量转换（成果2）、偶极化锋面前端的磁场南向偏移（成果3）、偶极化锋面后端能量离子的各向异性（成果4）。同时，本项目还分析了偶极化锋面与磁场重联之间的紧密联系（成果5），并分析讨论了在磁场重联过程中偶极化锋面的镜像结构，即重联点远地侧的反偶极化锋面及其粒子特性（成果6）。这些工作通过分析磁尾等离子体的行为特点，大大增强了人们对地球磁尾物质与能量输送过程的理解。..其次，本项目还着力分析了磁尾粒子注入内磁层后对内磁层的影响，即内磁层中的等离子体波动。在等离子体自由能驱动波动的同时，这些波动也可与特定能量的带电粒子发生共振作用，从而造成了地球磁层中丰富多彩的粒子行为。我们的工作从理论、模拟和卫星（包括美国Van Allen Probes卫星和我国北斗导航卫星）数据分析等多角度出发，完善和发展了超低频波与带电粒子的相互作用理论（见成果7-12）。